In recent years, there have been great needs of communication systems and sensing systems which employ the high frequencies of milliwaves, etc. Especially in a milliwave sensing system, a milliwave imaging sensor capable of sensing the shape of a target in all weathers has been greatly needed. The milliwave imaging sensor is classified into an active type wherein the milliwaves are transmitted to the target, so as to sense the target shape on the basis of the reflected waves of the milliwaves, and a passive type wherein the target shape is sensed on the basis of the milliwaves radiated by the target or from surroundings. It is known that, although the passive type milliwave imaging needs to detect radio waves feebler (i.e., weaker) than in the active type, it is higher in imaging resolution than the active type.
Now, the principle of the passive milliwave imaging will be outlined.
According to the Planck's law of radiation, any object radiates an electromagnetic wave which is determined by the temperature of the object, and an emissivity that depends upon the material of the surface of the object and an angle defined between a surface bearing (i.e., surface orientation) and a radiation bearing (i.e., radiation orientation). The radiated electromagnetic wave has its peak power in the region of infrared light, but it has feeble radiation also in the radio-wave bandwidth of a milliwave band and a microwave band. Radiation power P [W] in the milliwave band can be expressed as P=kΔf(∈T)[W] (hereinbelow, termed “Formula 1”, which is Rayleigh-Jean's approximate formula). Here, k[J/K] denotes the Boltzmann's constant, Δf [Hz] denotes an observation bandwidth, T [K] denotes the physical temperature of the target, and ∈ denotes the emissivity.
Recently, there are great needs of the passive type milliwave imaging sensor in which the shape of the object is recognized by receiving the radiation power in the milliwave band. It is known that, as shown in FIG. 20, the milliwave is higher than visible light in a transmission factor in a mist. FIG. 20 shows a relationship between frequency and attenuation constant in mist atmosphere. By way of example, the transmittivity of the milliwave is stated on page 207 in “Fundamentals and Applications of Milliwave Technology” issued by Kabushiki-Kaisha Realize Inc. (issued on Jul. 31, 1998, first edition, “Fundamentals and Applications of Milliwave Technology” Editing Committee). By reason of the high transmission factor, the passive type milliwave imaging sensor is greatly expected as the imaging sensor which is not influenced by the weathers.
At present, a method employing a flat patch antenna and a sensing circuit as disclosed in Japanese Patent No. 3,263,282 and JP-A-6-331725 is known as to the sensing module of the milliwave imaging sensor. In addition, there is known a method employing a tapered slot antenna and a sensing circuit as disclosed in JP-A-10-332824, JP-A-11-163626 and JP-A-11-330846. The “tapered slot antenna” is such that a thin metal plate whose central part is cut off in a taper shape is stuck on a flat glass sheet. Such tapered slot antennas are arranged as an array, and are used as the imaging sensor.
Further, a structure wherein a waveguide horn and a flat antenna are combined is known as disclosed on pp. 1473-1482 in “IEEE Transactions on Antennas and Propagation”, Vol. 38, No. 9, September, 1990. With the structure, a membrane (thin film) of silicon oxide is disposed perpendicularly to the propagation direction of the milliwave, within the horn antenna, and the flat antenna is located in a place where the membrane floats relative to the horn antenna, so that reduction in a depthwise dimension is possible.
The flat patch antenna has a broad directivity. Therefore, when it is employed in combination with a lens, it exhibits the directivity in an unnecessary range. Besides, it has a narrow band, and the reception power intensity thereof depends upon the band (Formula 1). Accordingly, the flat patch antenna has the problem that it is unsuitable for the milliwave imaging.
Besides, the tapered slot antenna has a directivity of end fire type, and it features the presentation of the directivity in a direction horizontal to a substrate. Such a structure poses the structural problem that the shape of the module cannot be made small in the depthwise direction thereof.
Further, the structure wherein the membrane is floated within the horn is structurally complicated and is low in strength. Therefore, performances disperse among elements, and imaging or the like in which uniform performances are required for all the elements is difficult of realization. Besides, since a bolometer is employed for the flat antenna, the structure is less immune against ambient temperature changes.